Fingerprint sensing is now widely used for identification or verification purposes. For this, a person's fingerprint is acquired by a fingerprint sensing device whose output is processed and compared with stored characteristical data of one or more fingerprints to determine whether a match exists. Most fingerprint sensing employs an optical imaging technique, as illustrated schematically in FIG. 1. Light from a light source 12 is directed into a glass prism 10 via its face 10a onto a second prism face 10b which provides a platen 11 upon which finger(s) are placed, then by total internal reflection (TIR) the light reflected from the second prism face 10b passes through a third prism face 10c, is redirected via an optional field lens 13a and is then imaged by an objective lens 13b onto a two-dimensional (2D) sensor 14 (see e.g., U.S. Pat. Nos. 2,195,699 and 5,416,573). The field of view (FOV) of the objective lens 13b with aperture stop 16 is bound by the dash-dot-dash lines of 9a and 9b. Light coming from prism 10 that is inside this FOV will be imaged by the objective lens 13b towards the sensor 14 and light outside said FOV will not make it through the aperture stop and/or objective lens.
In using this optical imaging technique ambient light 18 is protected from being detected by sensor 14, in part, through three optical principles: imaging of total internal reflection (TIR) light; aperture stop 16 of the objective lens; and/or spectral filter 17. Since only TIR light is being imaged, outside light (i.e., ambient light) normally would not be imaged. For example, consider ambient light 18b which enters prism 10 via face 10b. After transmitting through prism 10 and exiting prism face 10a it is specularly reflected off of light source 12 and enters back into prism 10, reflects off of prism face 10b, emerges out prism face 10c and heads towards aperture stop 16 as ray 15b. Since all the reflections of the ambient light in the aforementioned example are specular, ray 15b is outside of the imaging system's FOV and said ray is therefore not detected by sensor 14. Alternatively for example, consider ambient light 18a that follows the same entry path as ray 18b, where ambient light 18a is scattered by light source 12 into dashed ray 19. Because the propagation angle of scattered light 19 is different from specular light, it is able to reflect off of prism face 10b and exit prism face 10c as ray 15a which is within the FOV of the system's aperture stop 16 and objective lens 13b. 
Once light is imaged by the objective lens, it can still be blocked by a spectral filter 17. By way of example, if the illumination system for scanning fingerprints operates using 525 nm LEDs, such spectral filter may be a bandpass filter that passes only light between 500 and 550 nm. In this manner, red, infrared, and blue light comprising ambient light (e.g., such as that from overhead fluorescent light bulbs or the sunlight coming in through a window) is blocked and will not affect the light receiving (pixel) elements of the sensor 14.
Optically sensing of a fingerprint may also be performed using a photoelectric sensor 20 such as described in U.S. Pat. No. 5,991,467 or 7,369,690 which are incorporated herein by reference and shown schematically for example in FIG. 2. Backlight illumination 21a from a source 21 is transmitted to strike a finger 22 that is placed on a platen 23 and then light reflecting/scattering off of the finger 22 is detected by a two-dimension array of light sensitive detectors 24. The light sensitive detectors 24 each have a capacitor or capacitance which stores the accumulated charge of the detector 24 in accordance with the amount of the reflected light 25 the detector 24 receives. The amount of light received 25 into each of the light sensitive detectors 24 differs according to its position from which the light is reflected because a reflectance between a light reflected 25 from a ridge 8 portion that is protruded portion of the finger 22, and a light reflected from a valley 7 portion that is recessed portion of the finger 22, is different from each other, where the ridges and valleys of FIG. 2 have been drawn in an exaggerated scale in order to clarify the operation of the device.
Transistors 26 are provided for each of the detectors 24. Each transistor 26 switches to readout out the amount of the electron charge stored in the capacitor of its associated detector 24. The switching transistors 26 may be thin film transistors known as TFTs and light sensing detectors 24 may be thin-film based PIN photodiodes,
Platen 23 may be provided by the surface of a thin protective layer 27 over a substrate or transparent backplane 29 having detectors 24, and other electronics, including transistors 26, electrical connections, and other elements, typical of TFT-based sensors for enabling their operation. Fabrication of sensor 20 may use amorphous silicon technology formed on a backplane 29 of glass. Backlight illumination 21 passes through substrate 29 and the non-opaque areas (e.g., areas that do not contain detectors 24, transistors 26, electrical connections and other elements) of substrate 29. Detectors 24 are opaque on the side facing substrate 29 so that illumination light 21a from source 21 cannot be directly detected, but only detected because of a reflection or scattering from the front side of sensor array 20a. 
Detectors 24 are referred to hereinafter as light sensing pixel elements (or pixels) 24 of the two dimensional sensor array 20a, since each detector senses light in accordance with one pixel (when readout by other electronics on the chip of sensor 20) of a two-dimensional image representative of a fingerprint of the subject finger 22 or finger(s), palm, thumb, or other skin topology of a person. Since the finger 22 is in close proximity to the light sensing pixels of array 20a, no imaging optics are used (e.g., no objective lens, or other optics for focusing or magnification, and hence magnification of the light onto the array is one-to-one (1:1)). Thus, the term of a device using this photoelectric sensor to capture a fingerprint image is referred herein as a non-imaging contact sensor 20, where such sensor has a two-dimensional sensor array 20a of light sensing pixels. Fingerprint contact sensors where TFTs provide transistors 26 are referred to herein as TFT-based fingerprint contact sensors. However, heretofore the improvements provided by the present invention, a commercially useful non-imaging contact fingerprint sensor has not been successfully developed for use in fingerprint scanners. Such being desirable since avoiding the need for imaging optics of a fingerprint scanner of FIG. 1 would enable the scanner to be more compact and lightweight, especially useful for mobile fingerprint scanners.
For a fingerprint sensor that is based upon a non-imaging contact approach as depicted in FIG. 2, one does not necessarily have those same principles with which to block ambient light 28. No TIR effects are utilized and since no imaging optics are used; there is no aperture stop to limit the field-of-view of light striking the light sensing pixels. Because of these facts, any of the light sensing pixels 24 of array 20a that are not directly underneath portions of the finger 22 placed on the platen 23, said light sensing pixels will be exposed and typically saturated by ambient light 28. Even the light sensing pixels of the sensor that are directly underneath the finger may also be affected by ambient light. The light sensing pixels 24 are exposed by ambient light 28 due to light striking pixels not shadowed by the finger and scattering off of switching transistors 26, light sensing pixels, or other electronics on the chip and scattering into the areas where the light sensing pixels 24 would normally be shadowed by the finger 22. Therefore, although at first glance, one might believe that due to the thin protective layer 27 (sometimes just a few microns of SiO2 to protect the amorphous silicon) the finger will shadow at least the light sensing pixels 24 directly beneath, but this is not the case. It has been found that fingerprint images taken with a TFT-based non-imaging contact sensor of the type depicted in FIG. 2 with the room lights “off” provides an adequate fingerprint image, however when ambient light provided by room lights that are “on” is present, a significant amount of fingerprint image becomes saturated and lost. Such saturation worsens for fingerprint contact sensors that operate with outdoor ambient illumination, rather than room (artificial) light ambient illumination, since sunlight as generally significantly higher intensity than room light. It would thus be desirable to avoid this problem of saturation due to ambient light on non-imaging contact fingerprint sensors.